Magnetic devices

ABSTRACT

Partial substitution of small amounts of cobalt may result in a reduction in size of &#39;&#39;&#39;&#39;bubble&#39;&#39;&#39;&#39; magnetic domains in a variety of canted antiferromagnetic materials exemplified by rare earth orthoferrites. The reduction in domain size is ordinarily accomplished by virtue of a shift in the magnetic reorientation temperature, permitting improved device operation in this temperature region.

United States Patent Gyorgy et al. Mar. 7, 1972 MAGNETIC DEVICES References Cit d [72] Inventors: Ernst M. Gyorgy, Madison; Richard C. UNITED STATES PATENTS New Le 2 996 457 s 1961 s bod .25 a 51 Van MOlTiS Township, all Of Morris a n County, NJ

[73] Assignee: Bell Telephone Laboratories, Incorporated, Primary Examiner-James E. Poer Murray Hill, Berkeley Heights, NJ. Assistant Examiner-J. Cooper [22] Filed: No 17, 1969 Attorney-R. J. Guenther and Edwin B. Cave [21] Appl. No.: 877,369 [57] ABSTRACT Partial substitution of small amounts of cobalt may result in a .340/ 174 340/ l 74 SR, 252/62-56, reduction in size of fbubble magnetic domains in a variety of 252/6257, 252/6259 canted antiferromagnetic materials exemplified by rare earth [51] Int. Cl. ..Gllb 5/62 orthoferrites The reduction in domain size is ordinarily [58] held of Search "340,174; complished by virtue of a shift in the magnetic reorientation temperature, permitting improved device operation in this temperature region.

4 Claims) Drawing Figures i BACKGROUND OF THE INVENTION 1. Field of the Invention The invention is concerned with magnetic materials suitable for use in any of a class of devices utilizing small enclosed regions of opposite polarization variously known as bubble domain devices or single-wall" domain devices. Devices in this class depend for their operation on the nucleation and propagation of such domains, their presence and/or position representing information bits.

2. Description of the Prior Art The general nature of bubble domain devices and some description of many of the forms that such devices may take is set forth in The Bell System Technical Journal, Volume XLVI, No. 8, Oct. 1967, p. 1901-1925. The appeal of such devices is based on a number of characteristics including ease of the write and read functions, small power requirements, and on bit density. It has been estimated that with present circuit capability, bit density may appreciably exceed that of one bit per 100 square mils. This latter represents the present day general operating limit on materials that are otherwise suitable, e.g., available in sufficient size and sufficiently high quality to meet some contemplated device designs.

At this time one of the more promising classes of materials is the rare earth and related orthoferrites having the formula A20 in which A is a rare earth, lanthanum, yttrium or bismuth ion, and Z is commonly trivalent iron. Many of these materials, for example, terbiurn orthoferrite, readily support bubble domains of the order of 3 mils in diameter at operating temperatures near room temperature. Other desirable device properties of these materials notably include the velocity at which bubbles may be caused to progress from one position to another at desirably low-power levels.

Recognizing the desirability of these exemplary materials, practitioners have attempted to modify them in a number of ways to reduce stable domain dimensions so as to increase bit density. The magnetic nature of the orthoferrites is such that one approach, at least in retrospect, seemed to be virtually self-evident. At some temperature, sometimes denoted the spin-flop" temperature, although probably more properly referred to as the magnetic reorientation temperature, T,, the easy direction of magnetization switches by 90 from one axis to another in the rectilinear structure of the material. It has been known for some time that stable domain size decreases as T, is approached (and anisotropy consequently decreases).

Typical rare earth orthoferrites have values of T, which are characteristically at least 100 C. below room temperature. Only samarium orthoferrite has a value of T, which is substantially above room temperature. Attempts to produce materials supporting reduced bubble domains at usual operating temperatures have included a blending of samarium orthoferrite together with other members of the class. In fact, such blending does result in a value of T, intermediate that of the end members and, in fact, this has resulted in a reduction in domain dimensions.

While the blended samarium-containing materials continue promising for device use, they are possessed of an unfortunate characteristic which limits their application. The attainment of appropriate values of T, for usual operating temperature gives rise to the need for relatively large amounts of samarium (amounts of the order of 50 atom percent based on the total rare earth are characteristic). Since the effect of samarium addition is also to decrease the magnetic moment in most instances, and since reduction in moment tends to increase domain size, a significant reduction is achieved only at operating temperatures quite close to T,. This in turn gives rise to a strong dependence of domain size on temperature. While devices of increased bit density can be made to operate in stable fashion by close control of temperature, it is clearly desirable to somehow produce a bubble reduction while minimizing the need for close temperature control.

SUMMARY OF THE INVENTION In accordance with the invention,it has been determined that stable bubble domain size may be reduced in a family of magnetic materials exemplified by the rare earth and related orthoferrites by use of relatively small cobalt substitutions. These substitutions, which are generally less than 20 atom percent based on total Z-ion content (iron in the instance of the orthoferrites), permit a decrease in bubble dimension while 0 having a relatively small effect on magnetic moment.

For the example of the rare earth orthoferrites, the reduction in bubble dimension is analogous to that of samarium addition, with such partial substitution resulting in an increase in T, from low temperatures to temperatures more closely approaching operating temperature near room temperature. For all of the usual materials, this result is obtained with a lesser effect on magnetic moment, thereby permitting device operation further from T, and lessening the temperature dependence.

A preferred embodiment of the invention is accordingly defined as above, i.e., improvement for device use results from an increase in T, in the rare earth orthoferrites. A still more preferred embodiment is defined in terms of those compositions in which this increase is obtained while maintaining the magnetic moment at a value higher than that obtained by use of samarium. For the usual situation where operation within 50 of room temperature is contemplated, this implies the use of such orthoferrites which have magnetic moments higher than that of samarium. These preferred orthoferrites are those in which the cation is in order of decreasing moment for the related orthoferrite chosen from the group: Ho, Yb, Tm, Tb, Dy, Er, Lu, Y, Gd, La and Eu. The positions of Ho and Er, are those for alignment of the moment along the A-axis of the orthoferrites as a result of incorporating cobalt.

From the above grouping, it is evident that by rare earth is meant not only the members of the 4] group of the periodic table, but also the commonly associatedelements Y, La and Bi.

In a broader aspect, the invention embraces the use of cobalt to reduce magnetic anisotropy in a larger class of materials which either exhibit or may be made to exhibit uniaxial magnetic anisotropy by stressing, magnetic annealing or by other means. This broader class of materials may be defined in accordance with the appropriate chemical formula AZX or multiples thereof in which A, in addition to the rare earths as abovedefined, may be Y, La, Bi, Ti, V, Cr, Mn, Fe and Ni and mixtures thereof. Additionally, the A-site ions may be partially replaced as described in the detail description.

In the generalized formula, 2 may be B, C, Mn, Ni, Ti, V or Cr, in addition to Fe and mixtures thereof, The X-ion is oxygen, fluorine or sulphur, and n, the multiplicity divider, is unity in the case of the monovalent ion, fluorine, and two for use of the divalent ions, oxygen or sulphur.

Referring to the generalized class of substances upon which the invention may beneficially be practiced, reductions in bubble size invariably result from a reduction in magnetic anisotropy. In many cases the reduction in anisotropy is temperature-dependent as where T, is approached. in others, no such relationship is apparent, it being observed only that anisotropy and attendant bubble size is in fact reduced.

While the preferred embodiment has been defined in such terms as to result-in decreased temperature dependence, as compared with samarium addition to the orthoferrites, a more general form of the invention also contemplates cobalt substitution even where such improvement, as compared with samarium, is not realized. Such embodiments might desirably be practiced for a host of reasons, including, for example, possible ease of fabrication whether growth be massive or layered, etc.

The procedure for preparing the materials of the invention is not critical as the effect of cobalt on uniaxial anisotropy is mechanistic in nature and applies to vapor deposited, sputtered or melt grown materials as well as flux grown crystals. However a prevalent method of growth of orthofem'tes, for example, is from the flux.

As an example YFe Co O can be grown from the mixture, in a covered platinum crucible by use of the ingredients:

Y,0, l6 g. Fe,0 l l .3 g. Co,0, 0.08 g. Per, 80 g. PbO 70 g. 8,0, 3 3.

either by evaporation of the flux at l,300 C. or by slow cooling the melt to 900 C. The distribution of Co between melt and crystal is usually considerably less than unity.

BRIEF DESCRIPTION OF THE DRAWING FIG. I on coordinates of temperature in degree Kelvin and atom fraction of cobalt (in terms of total Z-ion) is a plot of the relationship of these parameters for two transition temperatures, T, and T (Curie temperatures); and

FIG. 2 is a schematic representation of a device depending for its operation upon the nucleation and propagation of single-wall domains in a material of the invention.

DETAILED DESCRIPTION 1. Composition The included compositions have been briefly outlined in an earlier section. Preferred and more general embodiments have been outlined. Additional variations on the general embodiment, of course, include combinations of the listed ions in the appropriate sites. Additional partial substitutions are also permitted. For example, up to 50 atom percent of any of the elements Sc, Th, U, Np, Pu, Mg, Co, Ba, Sr, Zn, Cd, Hg, A1, Ga, In, Tl, Si, Ge, Sn, Pb, P, V, As, Nb, Sb, Ta, Mo, W may be included in the A or Z site or Se, Te, Cl, Br or I may be substituted in anion positions. In this and all other descriptive material herein contained, ion percent, atom percent, or cation percent is invariably based on the total number of ions in the concerned site.

The general category of magnetic compositions includes rhombohedral structures such as FeF those of the calcite type such as FeBO and those of the ilmenite type such as FeTiO Other materials of natural or induced uniaxial anisotropy which are included are aFe o Fe BO YFeO and GaFeO A still more preferred embodiment in terms of composition is the cobalt-modified class based on orthoferrites wherein a major portion (more than 50 atom percent) of the A-ions are Yb, Tm, Ho, Er, Dy or Tb. This preference is based in large part on the magnetic moment which can be improved in this class of substances by use of the particular included cations. As noted, maximizing this parameter permits operation in a less temperature-dependent region for those substances in which improvement results by reason of an increase in T,.

It has been noted that cobalt substitution is generally less than 20 atom percent. A preferred cobalt range is dependent to some extent on composition and sometimes on operating temperature (in the instance of substances in which improvement is produced by approaching T,). A minimum cobalt content of 0.01 atom percent is set by the observation that lesser quantities do not result in a marked improvement in the magnetic material. A preferred range is defined as from 0.02 atom percent to 10 atom percent. A still more preferred range is defined as from 0.03 atom percent to 5 atom percent. In each instance, the minimum inclusion results in a desired minimal degree of improvement, while the maximum inclusion is set by the observation that further increase in amount generally results in diminishing improvement.

2. Mechanism The orthoferrites specifically, and many of the included materials, are canted-spin,.antiferromagnets. In terms of the orthorhombic case, either naturally occurring or induced (as by strain), the net magnetization is along either the A-axis or the C-axis. Such materials frequently undergo a magnetic reorientation at a temperature T, at which time the canting is switched from one axis to the other. At this temperature, the uniaxial magnetic anisotropy as between the A- and C-axes is reduced to zero. Other relevant quantities such as magnetic exchange and magnetic moment remain substantially unchanged when passing through T,.

The minimum size bubble domain D that can be supported in a magnetic medium of appropriate uniaxial anisotropy tends to vary in accordance with the equation:

D==K,, "/M in which K, is the uniaxial magnetic anisotropy as between the C- and A-axes and M is the magnetic moment.

The advantage noted above for cobalt substitution as compared with samarium substitution is readily seen. The lesser cobalt content required to attain a particular value of T, results in a final composition of greater moment, M. Since the relative variation in bubble size D is inversely as the second power of M, and directly, only as the one-half power of K, even a small increase in M permits a significantly increased anisotropy, so in turn permitting operation in a less temperature-dependent region.

The above relationship of course applied regardless of the mechanism responsible for the reduction in K,,. Since the observed general effect of cobalt addition is to reduce the magnitude of this parameter while having only a minimal effect on M, such addition invariably results in a reduction in bubble dimension.

3. FIG. 1

This figure, which is merely illustrative, indicates the very small necessary inclusion of cobalt required to increase T, in a typical preferred orthoferrite composition. It is seen that substitutions far below 1 percent are generally effective in increasing T to above room temperature. It is also seen that the efiect on T, is minimal and so it is assured that the composition continues to operate as a canted antiferromagnetic. While the curves on this figure are plotted specifically for the illustrative orthoferrites, the form of the relationships is found to be generally applicable to the preferred class of compositions herein, i.e., to all of the rare earth orthoferrites as defined.

4. Contemplated Device Uses It is considered beyond the necessary scope of this disclosure to describe bubble devices in their many forms in any detail. Such descriptions are available in the literature. See, for example, The Bell System Technical Journal, Volume XLVI, 1967, at pp. 1,90l-l,925. The device of FIG. 2 is merely illustrative and depicts a fairly simple configuration which may be considered as a portion of a larger assembly. In FIG. 2, a'register 10 comprises a sheet 11 of samarium orthoferrite in accordance with the invention. The sheet is so oriented that at the operating temperature the preferred magnetization direction (easy direction) is normal to the plane of the sheet. Flux directed out of the paper as viewed is represented by a plus sign. Flux directed into the paper is represented by a minus sign. Conductors l2, l3, and 14, which may be deposited on the surface of sheet 11, form triplets of loops 12a, 13a, 14a; 12b, 13b, 1412, et seq. Loop size is somewhat smaller than the size of a corresponding stable single-wall domain so that in operation any magnetized domain is partly within an adjoining loop. Such domains, once nucleated, for example, by means of a domain nucleating source 15 and loop 16, are stepped from loop position 12a to 13a to to 12b and so forth by successive energization of conductors l2, l3, and 14 in that order by means not shown. Readout is accomplished by means of loop 17 and sensing means 18.

Other device uses include switches, other types of memory elements, logic elements, etc. Some such devices may operate at constant temperature at or near the reorientation temperature. Others may depend on a temperature variation, sometimes local, to reverse the magnetization and so provide a means for easily nucleating a domain.

In other manner, the device description has been rudimentary. Devices of the type depicted in FIG. 2 have been developed to a far more advanced state. Some no longer utilize looped conductor configurations but depend upon the flux concentration which results from a sharp turn in the conductor pattern. A simple zigzag pattern, for example, results in a bit location at each conductor reversal position. More generally, while present interest largely centers on the use of the materials of this invention in single-wall domain elements, other devices may depend upon more conventional properties such, for example, as overall changes in magnetization, in changes in transmission properties for electromagnetic energy, under the influence of an applied field or with temperature change, etc.

What is claimed is:

1. Device consisting essentially of a region of magnetic material of uniaxial magnetic anisotropy together with first means for producing a magnetic field across at least a portion of said region so as to effect a local reversal in magnetic polarization thereby resulting in a single wall domain evidencing a magnetic polarization opposite to that of adjoining portions of the said region together with second means for propagating said domain through at least a part of the said region, in which the said material is a rare earth orthoferrite composition of the approximate stoichiometry AFeO, in which A is at least one element selected from the group consisting of the 4f rare earth elements, Y, La, and Bi, characterized in that cobalt is included as a partial substitution for Fe in the above recited stoichiometry in the range of from 0.01 to 20 ion percent based on the total Fe ion content.

2. Device of claim 1 in which A is selected from the group consisting of Yb, Tm, Ho, Er, Tb, Dy, Lu, Y, Gd, La, and Eu.

3. Device of claim 2 in which the iron is partially replaced by cobalt within the range of from 0.02 ion percent to 10 ion percent.

4. Device of claim 3 in which the iron is partially replaced by cobalt within the range of from 0.03 ion percent to 5 ion percent. 

1. Device consisting essentially of a region of magnetic material of uniaxial magnetic anisotropy together with first means for producing a magnetic field across at least a portion of said region so as to effect a local reversal in magnetic polarization thereby resulting in a single wall domain evidencing a magnetic polarization opposite to that of adjoining portions of the said region together with second means for propagating said domain through at least a part of the said region, in which the said material is a rare earth orthoferrite composition of the approximate stoichiometry AFeO3 in which A is at least one element selected from the group consisting of the 4f rare earth elements, Y, La, and Bi, characterized in that cobalt is included as a partial substitution for Fe in the above recited stoichiometry in the range of from 0.01 to 20 ion percent based on the total Fe ion content.
 2. Device of claim 1 in which A is selected from the group cOnsisting of Yb, Tm, Ho, Er, Tb, Dy, Lu, Y, Gd, La, and Eu.
 3. Device of claim 2 in which the iron is partially replaced by cobalt within the range of from 0.02 ion percent to 10 ion percent.
 4. Device of claim 3 in which the iron is partially replaced by cobalt within the range of from 0.03 ion percent to 5 ion percent. 